The polymerization of conjugated dienes such as 1,3-conjugated dienes to form elastomeric homopolymers and copolymers utilizing various initiator systems is known. For example, such polymerizations can be initiated with organometallic compounds wherein the metal is a Group I metal such as lithium. These polymers and copolymers of conjugated dienes are useful for tire rubbers, molded rubber goods, molding compounds, surface coatings, etc.
Various organometallic compounds have been described in the literature as useful in the polymerization and copolymerization of conjugated dienes. Among the catalysts which have been proposed are various alkali metal acetylides. For example, U.S. Pat. No. 3,303,225 describes the use of metalated 1-acetylenes as active catalysts in the polymerization of vinylidene-containing monomers. Alkali metal acetylides are prepared by reacting an organo alkali metal compound with an acetylene under conditions to effect step-wise replacement of, first, the acetylenic hydrogen atom, and, second, the hydrogen atoms attached to the carbon atom which is alpha to the acetylenic linkage.
The stereopolymerization of butadiene and styrene in the presence of acetylenes and ketones is described by H. E. Adams et al, in Kautschuk und Gummi. Kunststoffe 18. Jahrgang, pp. 709-716, Nr, 11/1965. The authors studied the reaction of 1-butyne with 1, 2 and 3 moles of n-butyllithium in hexane, and the use of the materials obtained from such reactions as catalysts. The reaction of 1-butyne with one mole of n-butyllithium resulted in the formation of a white precipitate where the acetylenic hydrogen was replaced by lithium. When a second mole of n-butyllithium was added slowly to the reaction mixture, the white precipitate dissolves and the product is a clear lemon-yellow solution. Upon standing at room temperature, the solution becomes cloudy, and after about 210 hours, the precipitation of a yellow solid is complete. The product was identified as 1,3-dilithio-1-butyne. When an excess of n-butyllithium is added to the precipitate of 1,3-dilithio-1-butyne, the precipitate dissolves to form a golden-yellow solution. There were signs of precipitation after two weeks, and after two months, a copious precipitate had formed. The precipitate is identified as a complex of 1,3-dilithio-1-butyne and n-butyllithium.
The use of dilithium salts in the polymerization of butadiene is reported by Makowski et al, J. Macromol. Sci.--Chem., E2(4) pp. 683-700, July 1968. Among the lithium compounds studied were the 1,3-dilithioacetylides such as the compounds obtained by reacting 1-hexyne with n-butyllithium in ratios of 0.5 and 0.67. At a ratio of 0.5, homogeneous catalyst solutions in hydrocarbons were obtained. Above this ratio, some precipitate was present. In all cases, however, polymerization with butadiene resulted in low molecular weight polymer solutions. That is, where the catalyst solution included precipitated solids, the solids dissolved during the course of the polymerization. At the ratio of 0.5, the polymer solution was very viscous, and at the ratio of 0.67 a gelled solution resulted. However, when Attapulgus clay was added to the viscous solution or to the gelled solution, fluid solutions were obtained. This result was attributed to the presence of water in the clay.
Polylithium polymerization initiators also are described in U.S. Pat. No. 3,377,404. The initiators are prepared by first contacting an excess of lithium with an organic halide containing two to four halogen atoms in a polar solvent such as ether. The intermediate formed in this step can be represented by the formula EQU RLi.sub.x
wherein x is an integer of two to four and R is a hydrocarbon group. In a second step, the intermediate is contacted with a small amount of a conjugated diene such as 1,3-butadiene. The amount of diene is generally from about one to about ten moles per mole of lithium compound. After the intermediate has been treated in this manner, a substantial amount or all of the polar solvent is removed and replaced by a hydrocarbon solvent. The polylithiated hydrocarbon soluble compounds prepared in this manner are reported to be useful as initiators of the polymerization of conjugated dienes including the polymerization of a mixture of a conjugated diene and other monomers such as vinyl-substituted aromatic compounds.
U.S. Pat. No. 3,784,637 describes multi-functional polymerization initiators prepared from polyvinylsilane compounds or polyvinylphosphine compounds. More particularly, the multi-functional polymerization initiators are prepared by reacting an organomonolithium compound such as n-butyllithium with a polyvinyl phosphine compound or polyvinylsilane compound. Preferably, the reaction is conducted in the presence of a solubilizing monomer such as a polymerizable conjugated diene, monovinyl-substituted aromatic compound, or mixtures thereof. Examples of solubilizing monomers include conjugated dienes such as 1,3-butadiene and aromatic vinyl compounds such as styrene.
Elastomer compositions comprising a blend of styrene butadiene rubbers are described in U.S. Pat. No. 4,471,093. The blends which are useful in tire tread compounds comprise a mixture of a high molecular weight styrene-butadiene rubber having a specific microstructure and a low molecular weight styrene-butadiene and/or butadiene rubber. The high molecular weight styrene-butadiene rubber component is further characterized as having a styrene content of 15-35% by weight, a 1,2-bond content in the butadiene component of 40 to 70 mole percent, and an intrinsic viscosity of 2.0 to 6.0 in toluene at 30.degree. C. The ratio of the weight average molecular weight to the number average molecular weight is defined as being not more than 2.0.